Substrate pore size regulating the structure and performance of polyamide nanofiltration membranes for ion separation

Substrate pore size dictates monomer diffusion via capillarity. • 30 k Da substrate balances monomer transport & reaction, forming uniform sub-nm pores. • NF-30 k membrane achieves exceptional ion separation: S Cl , SO 4 $S_{\mathit{Cl},{\mathit{SO}}_4}$ = 108, S Li , Mg $S_{\mathit{Li},\mathit{Mg}}$ = 29.1. • NF pore distribution matching hydration diameters is the key to ion separation. • Membrane shows 240 h stability & superior Li/Mg separation in 5 simulated brines. Substrate properties influence polyamide (PA) formation during interfacial polymerization (IP), yet the relationship between monomer storage within the substrate and monomer replenishment toward the reaction interface remains unclear. Polyethersulfone (PES) substrates with different pore sizes were used to examine this relationship. Larger pore substrates have greater aqueous phase retention, while the apparent aqueous phase diffusion of piperazine (PIP) decreased with increasing pore size. The opposite trends indicate that greater monomer storage does not necessarily result in more effective monomer replenishment toward the reaction interface. Diffusion experiments and capillary analysis further show that substrate pore size regulates PIP diffusion and replenishment through capillary effects within pores filled with water, which alters the local monomer supply near the reaction interface and affects PA growth. The resulting PA layers therefore differed in thickness, morphology, and pore size distribution. Among the investigated substrates, PES-30 k provided a suitable balance between monomer replenishment and interfacial reaction, producing a more homogeneous sieving structure with a Li + /Mg 2+ separation factor of 29.1 and promising performance in simulated salt lake brines. This study provides new mechanistic insights into how substrate pore structure regulates interfacial polymerization and offers a mechanistic basis for the rational design of TFC nanofiltration membranes through substrate pore structure optimization.